1. Field of the Invention
The present invention relates to telecommunication systems conforming to the cdmaOne standard.
2. Description of the Related Art
The cdmaOne.TM. communication standard is an interim standard for mobile telecommunication systems in which communications to and from each mobile unit are supported by one of a set of base stations strategically located over the system coverage area. The cdmaOne standard specifies a common air interface for code division multiple access (CDMA) systems on the cellular (900 MHz) and the PCS (1900 MHz) bands for mobile telephony. In addition, the same air interface is used for different wireless loop equipment supplied by a significant number of manufacturers. The term "cdmaOne" is used to refer collectively to the IS-95, IS-95A, and IS-95B family of communication standards.
The cdmaOne standard specifies that the symbols encoded in both the forward-link signal transmitted from the base station to the mobile unit and the reverse-link signal transmitted from the mobile unit to the base station be interleaved in the signal stream. Interleaving is performed to make burst errors during transmission look like random errors that are separated from one another in the de-interleaved symbol stream. In that case, the decoder in a receiver can perform error correction to reconstruct the original symbol stream notwithstanding the presence of burst errors.
According to the cdmaOne standard, a mobile unit transmits reverse-link data on an Access channel at 4800 bps and four Traffic (or Fundamental) channels at 9600, 4800, 2400, and 1200 bps. Each frame in a reverse-link channel contains 576 symbols. At 9600 bps, each symbol occurs once per frame. At 4800 bps, each symbol occurs two times in a row; four times at 2400 bps; and eight times at 1200 bps. The data rates of 9600, 4800, 2400, and 1200 bps correspond to the set of four unpunctured rates under the cdmaOne standard referred to as Rate Set 1.
The cdmaOne standard also supports a second set of data rates referred to as Rate Set 2. In Rate Set 2, punctured convolutional codes are used to transmit data at 14400, 7200, 3600, and 1800 bps, corresponding to the unpunctured rates of 9600, 4800, 2400, and 1200 bps, respectively. By using punctured convolutional codes, the number of symbols per frame is maintained, and the interleaving structure for the four rates of Rate Set 2 is the same as the interleaving structure for the four rates of Rate Set 1.
The cdmaOne standard specifies the reverse-link interleaving process at the mobile unit by means of a table. FIG. 1 shows the order in which the 576 symbols of each un-interleaved frame of reverse-link data may be sequentially (or linearly) arranged within a matrix of 32 rows and 18 columns in the mobile unit. The symbols are written columnnwise, beginning with the first column on the left, successively from the top row to the bottom row. The interleaving process involves reading the rows of the matrix in FIG. 1 (left to right) in a specified order. For the Access channel at 4800 bps, the rows are read in the following sequence:
1179255211329319112772315312181026622143042012288241632 PA1 1234567891011121314151617181920212223242526272829303132 PA1 1324576891110121315141617191820212322242527262829313032 PA1 1526374891310141115121617211822192320242529263027312832 PA1 1921031141251361471581617251826192720282129223023312432
For the Traffic channel at 9600 bps (or 14400 bps, if the Rate Set 2 option is activated), the rows are read in the following sequence:
For the Traffic channel at 4800 bps (or 7200 bps, if the Rate Set 2 option is activated), the rows are read in the following sequence:
For the Traffic channel at 2400 bps (or 3600 bps, if the Rate Set 2 option is activated), the rows are read in the following sequence:
For the Traffic channel at 1200 bps (or 1800 bps, if the Rate Set 2 option is activated), the rows are read in the following sequence:
The cdmaOne standard specifies that the reverse-link interleaving process be implemented using a table-based procedure that can be implemented at a reasonable cost only in software.